Bisphenol A is a very important monomer in polymer synthesis. It is the key starting material for most epoxy materials, as well as most polycarbonates. Commercial output for bisphenol A on an annual basis exceeds 5 billion pounds.
Most often, bisphenol A is prepared by the acid-catalyzed condensation of phenol and acetone. For example, phenol (in excess) and acetone can be mixed and warmed to about 50.degree. C. A homogeneous catalyst such as hydrogen chloride (HCI) is passed into the mixture over an extended period of time. A promoter-agent such as a thiol-based compound is frequently required to achieve commercially-acceptable reaction rates and selectivity.
The bisphenol A product (or an adduct of bisphenol A and phenol) precipitates, and can be filtered off and washed with a suitable solvent like toluene or phenol. The solvent treatment removes the unreacted phenol, which is recovered for re-use. The bisphenol A product can then be subjected to a "first-stage" purification by a variety of techniques, e.g., recrystallization or melt crystallization.
While the use of homogeneous catalysts had been favored for preparing bisphenol A for many years, the use of a heterogeneous catalyst system has become the most popular technique. Most of the heterogeneous catalysts are ion exchange resins, e.g., sulfonated polystyrene resins which utilize divinyl benzene as a cross-linking agent. The heterogeneous catalysts systems provide important advantages. For example, they are non-corrosive (in contrast to HCI); they can be readily separated from the reaction mixture; and they are very amenable to continuous reaction operation.
Regardless of the catalyst system used in the preparation of bisphenol A, the purity of the product is generally extremely important. For almost all applications, the para, para-isomer of bisphenol A (sometimes referred to herein as "p,p'-bisphenol A") is highly preferred, in contrast to the ortho, para- and ortho, ortho-isomers. Thus, for the purpose of this disclosure, the degree of "purity" generally refers to the maximum level of p,p'-bisphenol A present in the product mixture, as well as the minimum level of other impurities present, such as cyclic dimers derived from bisphenol A, chroman-based compounds, spirobiindane compounds, and the like. As used herein, "selectivity" is a more specific definition than "purity", referring to the percentage of p,p'-bisphenol A present in a reaction product mixture rendered free of phenol, acetone, and water. As noted above, it is desirable that the proportion of p,p'-bisphenol A be as high as possible.
In the case of epoxy resins, the purity of bisphenol A is not particularly critical, since color and high molecular weight are not usually important requirements. Thus, a material containing about 95-98% by weight of the p,p'-isomer might be quite sufficient. (The primary impurity for such a material would be the ortho, para isomer).
However, the bisphenol A used to prepare most polycarbonates must be of ultra-high purity, e.g., greater than about 99.5% and preferably, greater than about 99.9%. Impurities in the bisphenol A could lead to colored products, which would render the material useless for applications such as glazing. Moreover, the presence of impurities can prevent the build-up in molecular weight which is required for most polycarbonate materials.
The reaction processes currently used on a commercial scale often provide an "intermediate" product which is 70% selective to p,p'-bisphenol A. The product is then subjected to one or more purification steps which eliminate almost all impurities, and raise the selectivity to greater than about 98%. Many of the impurities are eliminated by being re-exposed to reaction conditions, which result in their isomerization to p,p'-bisphenol A. As an example, the reaction effluent is often treated in an adduct crystallizer.
Adduct crystallization processes typically involve, as a first step, the distillation (under reduced pressure) of a product mixture of bisphenol A, unreacted phenol, unreacted acetone, water, and by-products, so as to remove the water, acetone, and a small portion of the phenol. The remaining liquid mixture is cooled, resulting in the crystallization of an adduct of bisphenol A with phenol, as described in U.S. Pat. No. 5,723,688. The adduct crystals are then separated from the mother liquor, and phenol is removed from the adduct, thereby yielding the bisphenol A. After multiple passes through the adduct crystallizer, the effluent is very pure, with a selectivity at or close to the desired level. Typically, the effluent is then passed to another purification system, such as a melt crystallizer, which eliminates substantially all remaining impurities (including the o,p and o,o' isomers). The selectivity of the product which exits the melt crystallizer is usually greater than about 99.9% p,p'-bisphenol A.
The final bisphenol A product possesses the desired level of purity and selectivity. However, the purification steps required to attain that objective--such as adduct crystallization--add a great deal of expense to the overall manufacturing process. It is estimated that at least 60% of the cost of most commercial bisphenol A operations is typically related to purification of the product in the reactor effluent. In addition to capital (e.g., equipment) costs, current purification procedures require considerable periods of processing time. Moreover, since new uses for polymers made with bisphenol A require ever-increasing levels of purity for the raw materials, there is presently very little flexibility in the purification steps. Furthermore, when high-quality parameters are coupled with requirements that products be prepared at lower cost, a manufacturer is faced with a serious dilemma.
It should thus be apparent that new methods for preparing bisphenol A would be welcome in the art. In U.S. Pat. No. 4,346,249 (H. Krabbenhoft), the preparation of bisphenol A by a reaction which involves p-isopropenylphenol is briefly mentioned. However, the patent does not provide any teaching as to the preparation of bisphenol A with a high degree of purity, and with high selectivity toward the para, para-isomer of the product.
New methods for preparing bisphenol A should result in a product with high purity and selectivity. The methods should also reduce or eliminate the need for extensive purification treatments of the "raw" bisphenol A product. It would also be desirable if the new methods could be carried out by using a heterogeneous catalyst system which does not always require the use of a promoter (sometimes referred to as a "co-catalyst"). Moreover, in some instances, it would be useful if the new methods did not rely on acetone as a starting material.